I used to shoot an 11 ga Charles Moore double, and really I could not tell a lot of difference between that and a 12 ga. IIWM, I would go for a 12 ga instead of a 10 or an 11 Ga. But that's just me. I don't really think you would gain a lot going to the larger bore, except recoil, all things being equal.
I think a .715 ball weighs in at approx 555 grains, and that is enough to flatten most anything you're liable to run across on this continent.
IIRC, it is pretty unusual to find trade guns in anything but 12 and 20 bore, and mostly they are in 20 bore. So there is your historical perspective.
Good luck with the decision. Let us know what you decide and pictures of the trade gun when you get it.
John